When a user listens to music with earphones or headphones, the sound resonates in ear canals of the user by sealing the ear canals with the earphones or the headphones. The resonance phenomenon causes the user to hear an unnatural sound or the like. To avoid such unnatural sound, there has been proposed a technology aimed to suppress the resonance phenomenon in the space formed by the ears and the earphones or the headphones.
For example, Japanese Patent Application Publication (KOKAI) No. 2009-194769 discloses a conventional technology for cancelling a peak (resonance peak) of a resonant frequency detected by earphones for measurement provided with a microphone. According to the conventional technology, a sound source signal is output from the earphones. While the earphones are placed in the ear canals, the microphone picks up sound to obtain the frequency characteristics of the acoustic signals. The resonant frequency of the ear canal is detected from the frequency characteristics to reduce the resonant frequency. Further, Japanese Patent No. 2741817 discloses a conventional technology for alleviating discomfort of the user when the headphones are placed and the ear canals are sealed. According to the conventional technology, transfer functions that are different for each individual are clustered to reduce a number of types of filter coefficients of a filter for controlling auditory lateralization. This eases the listener's setup burden.
However, the above conventional technologies require hardware for measuring the sound in the ear canals, thereby increasing the cost for providing the hardware. Further, typically, it is difficult to find such hardware. Still further, the above conventional technologies require a listening test for at least sixteen times to determine an appropriate filter coefficient. Hence, the setup of the filter coefficient is still complex. Further, because sometimes the difference between the different setups is very small, it is very difficult to recognize the difference between the sound qualities under the different setups.